Max tire size - 68 dart

I can run 275 on 18s up front on a 69 dart?

Don't see why not. The front fender clearance isn't any different than other model year Darts unless maybe you consider the trim studs that MuuMuu is having issues with. That's what I have on my Duster now and it has '71 Dart fenders. You need an 18" rim to clear the tie rods, for a 275/35/18 you need an 18x9" with about 6" to 6.2" of backspace (25 to 27mm offset) assuming you've got 73+ BBP disks. Most likely tubular UCA's as well to clear that deep of a backspace at full steering lock and suspension extension.